A phrase used on Internet forums to counter the vast range of unverifiable claims made by users. Often these claims involve personal brag-worthy accomplishments, extraordinary or rare sights/occurrences, and tales relating to alcohol or drug use.

Guy 1: Dude, I drank four bottles of bourbon last night, then hooked up with 4 chicks at the same time, then Donald Trump came to my house and gave me a million dollars!
Guy 2: Pics or it didn't happen.

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Pics or it didn't happen is used when someone makes a very far fetched statement about something. See example.

Teenager1: So brah ask me what I did over the weekend
Teenager2: ok....what did you do over the weekend?
Teenager1: I went over to this girls house and I fucked her and then her mom walked in on us and we had a threesome.
Teenager2: Pics or it didn't happen looser!